Wind energy has used for powering machinery since ancient times. Since then, the need to generate power from greener and renewable sources like the wind has become ever more urgent, and wind turbines have been developed for the production of electrical power. In spite of this, wind power has seldom succeeded in commercial terms, owing to the variability of the supply of wind over time and geography. Typically, wind turbines operating in areas with consistently high wind speeds tend to be the most commercially viable, but such sites are rare.
Different wind turbine designs have been developed for use in different scenarios and applications. For example, they may be classified according to whether the blades of the wind vane rotate about an axis of a shaft which is horizontally or vertically disposed. Horizontal axis wind turbines (HAWTs) tend to be more commonly deployed as they tend to be more efficient: this is a result of blade rotation in a direction perpendicular to the direction of wind flow so that they receive energy through the entire cycle during rotation. However, they suffer various disadvantages, not least in the sheer height, size and weight of the towers and the blades, which makes installation, operation and maintenance extremely costly. They also need careful positioning into the wind, are unlikely to work well in conditions where the wind is variable in speed and direction. Such wind turbines are also potentially disruptive, in the visual sense as well as to anything from wildlife, to the transmissions of radio signals.
Vertical axis wind turbines (VAWTs) are inherently less efficient as the blades receive energy from the wind for only a part of its rotation cycle during which it is “blown” forward. For much of the remaining part of the cycle, the blade rotates in a direction substantially against the direction of wind flow. This can be contrasted with HAWTs, in which the wind energy is captured by the blade throughout its cycle. This will be described in further detail below; suffice it here to say that a large part of the energy captured from the wind is typically lost due to drag when the rotor blade travels into the wind as it goes through its cycle. VAWTs nonetheless have the advantage of being capable of harvesting power from winds of lower and more variable speeds. They tend to be smaller and lighter, and can be deployed at lower heights, resulting in reduced conspicuousness, installation and maintenance costs. This allows them to be used in a greater variety of locations. With their otherwise advantageous characteristics, attempts have been improve the efficiency of VAWTs. By way of example, US 2007/0241567 describes the use of guides to bias or channel the wind onto the rotor blades, which enhances the turbine's use in a variety of locations regardless of wind direction. A commercially-available VAWT marketed under the name of “StatoEolien” by Gual Industrie of France (http://www.gualstatoeolien.com/English/defaultang.html) includes a similar guide device. Although this helps to increase the effect of the available wind on the rotor blades, the overall efficiency of output remains poor, as the wind energy captured by the blade is later “given up” when the blade backtracks against the wind during the later stage in its cycle. As may be expected, this is a factor seriously affecting the efficiency of the turbine in the generation of electrical power, especially in low wind velocity areas where VAWTs are deployed, where any such loss is especially keenly felt.
It would be desirable to improve the efficiency of wind turbines, especially VAWTs.